Different mechanisms of bicarbonate use affect carbon isotope composition in Ottelia guayangensis and Vallisneria denseserrulata in a karst stream

Abstract Macrophytes live in extremely different carbon environments compared to terrestrial plants. In response to bicarbonate dominated inorganic carbon in natural aquatic systems, the ability for HCO3− use has been developed in many macrophytes to alleviate a limitation by CO2 in aquatic systems. In the present study, all present macrophytes in a karst stream were investigated, and we found that seven of nine species were able to use HCO3− and two species at the source of the karst stream were restricted to CO2 use. Two different inhibitors were used to determine the HCO3− use mechanisms in Ottelia guanyangensis and Vallisneria denseserrulata, the only species coexisting from up- to down-stream. External carbonic anhydrase (CAext) mediated HCO3− use was present in O. guanyangensis up-stream, and then O. guanyangensis shifted to a combined mechanism of CAext and anion exchange (AE) mediated HCO3− use down-stream. In these two species, high HCO3− concentration induced the ability to use HCO3−, but high CO2 concentration inhibited this ability. From up-stream to down-stream, the decreasing CO2 induced increasing proportion of HCO3− use that led to changes in the δ13C in O. guanyangensis. However, a constant CAext and AE mediated HCO3− use caused a relatively stable δ13C in V. denseserrulata from river source to 330 m and then increased the proportion of HCO3− use at the stream outlet associated with the change in δ13C.